1. Field of the Invention
This invention relates to a speech parameter coding method and apparatus for use with a speech coding method and system for coding a speech signal with high quality at a low bit rate, specifically, at about 8 kb/s or less.
2. Description of the Prior Art
Various methods of coding a speech signal at a low bit rate of 8 kb/s or less are already known. One exemplary conventional coding method is CELP (Code Excited Linear Prediction), which is disclosed, for example, in M. R. Shroeder and B. S. Atal, "CODE-EXCITED LINEAR PREDICTION (CELP): HIGH-QUALITY SPEECH AT VERY LOW BIT RATES", Proc. ICASSP, pp. 937-940, 1985 (reference 1) and also in W. B. Kleijn et al., "IMPROVED SPEECH QUALITY AND EFFICIENT VECTOR QUANTIZATION IN SELP", Proc. ICASSP, pp. 155-158, 1988 (reference 2). According to the method, on the transmission side, a spectrum parameter representing a spectrum characteristic of a speech signal is extracted from the speech signal for each frame (e.g., 20 ms). Each frame is divided into subframes of, for example, 5 ms, and a pitch parameter representing a long-term correlation (pitch correlation) is extracted for each subframe from excitation signal in the past. Then, long-term prediction (pitch prediction) of the speech signal of the subframes is performed using the pitch parameter. In accordance with a residual signal obtained by such long-term prediction, a noise signal is selected from a codebook which is constructed from predetermined noise signals so that the error power between the speech signal and a signal synthesized using the selected signal may be minimized, and an optimal gain is calculated. An index representing the kind of the thus selected noise signal and the gain are transmitted together with the spectrum parameter and the pitch parameter.
An efficient quantizing method not only for a excitation signal but also for a spectrum parameter is significant in order to further reduce the bit rate of such CELP method.
In the CELP method described above, an LPC parameter which is found by an LPC analysis is quantized as a spectrum parameter. Scalar quantization is normally employed as such quantizing method, and a bit number of 34 bits per frame (1.7 kb/s) or so is required to quantize 10th order LPC coefficients. While it is necessary to reduce the bit number of a spectrum parameter as low as possible in order to reduce the bit number of the CELP method below 4.8 kb/s, if the bit number is reduced in this manner, then the sound quality is deteriorated accordingly. A vector-scalar quantizing method has been proposed as a method of quantizing an LPC parameter more efficiently and is disclosed, for example, in Moriya, "Transform Coding of Speech Using a Weighted Vector Quantizer", IEEE J. Sel. Areas, Commun., pp. 425-431, 1988 (reference 3). However, such vector-scalar quantizing method still requires a bit number of 27 to 30 bits per frame. Accordingly, a more efficient method is required for the reduction of the bit rate.
Further, since the number of bits necessary for quantization of a spectrum parameter is reduced, if a greater length is provided to frames, then it is difficult to represent a change in time of a spectrum well and the distortion in time is increased, which results in deterioration in sound quality.